High-speed networks are continually evolving. The evolution includes a continuing advancement in the operational speed of the networks. The network implementation of choice that has emerged is Ethernet networks physically connected over unshielded twisted pair wiring. Ethernet in its 10BASE-T form is one of the most prevalent high speed LANs (local area network) for providing connectivity between personal computers, workstations and servers.
High-speed LAN technologies include 100BASE-T (Fast Ethernet) and 1000BASE-T (Gigabit Ethernet). Fast Ethernet technology has provided a smooth evolution from 10 Megabits per second (Mbps) performance of 10BASE-T to the 100 Mbps performance of 100BASE-T. Gigabit Ethernet provides 1 Gigabit per second (Gbps) bandwidth with essentially the simplicity of Ethernet. There is a desire to increase operating performance of Ethernet to even greater data rates.
FIG. 1 shows a block diagram of an Ethernet transceiver pair communicating over a bi-directional transmission channel, according to the prior art. The transceiver pair includes a first transceiver 100 and a second transceiver 105. The first transceiver 100 includes a transmitter section 110 that receives digital data for transmission over a transmission channel 135. The first transceiver 100 also includes a receiver section 120 that receives data.
The transceiver 100 includes a digital to analog converter (DAC) for transmission, and an analog to digital converter (ADC) for reception. The hybrid circuit 130 is designed to reduce the level the transmit signal present in the receive signal path. The transmitter section 110 and the receiver section 120 are connected to a common twisted pair (transmission channel 135) causing some of the transmission signals of the transmitter section 110 to be coupled into the receive signals of the receiver section 120. The coupled signal can be referred to as an “echo” signal. The echo signal can include two separate primary components. The first component includes transmission signals due to the direct connection of the transmitter sections to the input of the receiver sections. The second component includes reflections of the transmit signal within the transmission channel.
The hybrid circuit 140 of the second transceiver 105 operates in the same manner as the hybrid circuit 130 of the first transceiver 100. The transmitter section 150 and the receiver section 160 of the second transceiver 105 operate in the same manner as the transmitter section 110 and receiver section 120 of the first transceiver 100.
An implementation of high speed Ethernet networks includes simultaneous, full bandwidth transmission, in both directions (termed full duplex), within a selected frequency band. When configured to transmit in full duplex mode, Ethernet line cards are generally required to have transmitter and receiver sections of an Ethernet transceiver connected to each other in a parallel configuration to allow both the transmitter and receiver sections to be connected to the same twisted wiring pair for each of four pairs. As a result, each of the four receivers typically suffers from echo signal interference.
FIG. 2 shows an Ethernet transceiver that includes an exemplary echo cancellation circuits. The transceiver includes a transmit DAC 210 for converting digital transmission signals to analog transmission signals. The analog transmission signals of the DAC 210 are coupled (in some cases through line drivers) to a transmission channel 260. A receiver section of the transceiver is also coupled to the transmission channel 260 for receiving desired signals of the transmission channel 260. The receiver includes the previously described hybrid circuit 220 which mitigates the effects of echo signals.
The analog output of the hybrid is sampled by a receiver ADC 230, generating digital samples. The digital samples are processed by digital processing circuitry 240. Some echo interference is still present after the signals (desired and echo) have been passed through the hybrid circuit 220, and received by the digital processing circuitry. Some prior art methods of additionally canceling the echo signals include MMSE (minimum means squared error) algorithms performing linear echo cancellation. However, noise and non-linear components caused by the echo signal cannot be cancelled with traditional DSP algorithms and methods leading to SNR loss in the receiver. Additionally, large amounts of echo signal at the receiver ADC reduces the dynamic range of the receiver ADC and constrains performance of the receiver ADC.
It is desirable to minimize the amount of echo signal at a receiver ADC of a transceiver, and to improve cancellation of the echo signal causing signal processing of the transceiver to be low noise and highly linear.